Why do we participate online?

In a capitalist society there is the assumption that people never do something for nothing. This is true, but the reward doesn't always need to be dollars, or even something tangible. In fact, studies have found that paying someone to do something often brings them less enjoyment than if they had done it for fun. Once payment is involved, it becomes a job, it's no longer about enjoying what you're doing. At least that's the psychological perspective... imagine if we could enjoy it and get paid? But the current reality is that many brand sites try to solicit user involvement by the promise of some reward, a chance to win a prize for example. But some of the most successful community involved sites offer no tangible reward for involvement.